The current practice for the manufacture of sintered silicon nitride ceramics includes the use of a refractory material container (typically formed of graphite) in which the silicon nitride article is placed before sintering. A setter powder of silicon nitride or boron nitride or a mixture thereof is poured into the container to cover the silicon nitride articles being sintered to protect them from thermal decomposition and reaction with the furnace materials. Without this protection, experience has shown that a silicon nitride article will decompose at 1800.degree. C. in part to silicon and nitrogen. Furthermore, if the will react with the carbon in the graphite to form silicon carbide, which is stable at 1800.degree. C. When setter powder is used, the powder itself decomposes due to the high temperature in the furnace and the powder reacts preferentially with the furnace material instead of the silicon nitride article.
The above described method of using setter powder has been found undesirable, however, because the powder must be cleaned off of the article after sintering. At times, the powder on a silicon nitride article sinters together or sinters to the article, thus, requiring excessive force to remove the sintered material. Sometimes the sintered material cannot be removed even with force, and the article must be scrapped.
Recycling of the setter powder is not without its problems either. Firstly, during use, there is inherent loss of setter powder (due to spillage, etc.), making the operation more costly. It has been known to lose as much powder in weight as the silicon nitride article itself. In addition, during recycling, of&:en the setter powder needs to be crushed and screened to break-up the sintered pieces before reusing.
Yttria or other sintering aids are sometimes added to the setter powder, because, if the silicon nitride article contains these sintering aids, the sintering aids are "robbed" from the article by the setter powder during sintering unless the setter powder also contains the aids. In instances when sintering aids are added to the setter powder, the setter powder typically is chemically analyzed to ensure that the sintering aids are present in the proper amounts.
It is the primary object of the present invention to provide a device which prevents silicon nitride articles from thermally decomposing or reacting with furnace materials during sintering.
It is another object of the present invention to provide a device which provides protective coverage to a silicon nitride article during sintering operations and which is easy to use, readily reusable without any additional manipulation of the device, and one which does not require any additional steps of operation, such as post-cleaning of the sintered article. It is yet another object of the invention to provide a protective vessel for containing reaction bonded silicon nitride parts, such as mechanical seals.